yep u guessed right , its is because it supports dual channel ram. it does only have 512kb of Level2 cache, but that doesnt give much of a disadvantage in performance. socket 939 is even faster than socket 940, because that requires registered and ECC RAM which is slower than conventional ram used by most people. I read an interesting article on this a few days ago, its quite long...

754: does not have dual channel ram support, goes up to 3700+(2.4Ghz)
939: does have dual channel ram support goes up to 3800+[currently (2.4Ghz)]. also note that the FX series can be bought for socket 939.
940: Opteron and FX series socket goes up to FX-53 (2.4Ghz) and the opteron of 2.4Ghz. is it called the "240?"